Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
FireShot Webpage Screenshots captures webpage screenshot entirely (see demo: http://bit.ly/RlBQMV)
The screenshot can be quickly annotated and:
+ Saved as PDF/PNG/GIF/JPEG/BMP
+ Sent to clipboard
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
Secure Login is an extension for Firefox integrated password manager.
Allows you to resize the window to specific dimensions.
NOTE: Users of Firefox 29 and above must install "The Addon Bar" extension to use this, as Firesizer uses the add-on bar that has been removed in Firefox 29.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Useful tools for the nightly tester.
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10
Gives a possibility to create custom toolbarbuttons...
simply turn on|off images on all web-pages and (optional) autoreload current...
Collections are groups of related add-ons that anyone can create and share.Explore Collections